Under-Roof and Attic Insulation — Insulation, HVAC

Under-Roof and Attic Insulation - Insulation, HVAC

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Cathedral ceilings can be insulated using fabric to hold blown-in insulation in place between rafters and against the underside of the roof.

Your roof is the point of contact between the house and the outside. Insulating the top space in the house creates a complex relationship between the conditioned space inside and the varying temperatures outdoors. Insulating the roof helps the house transition between temperatures and provides a measure of control over dangerous moisture buildup within the home.

Insulating Under the Roof

Insulating attic floors over a flat ceiling is the easiest way to insulate the space under the roof. These floors are typically accessible, with lots of room for insulation between the rafters. Insulating the attic roof puts any heat or air conditioning ducts within the conditioned space. How you deal with those ducts should be discussed with a professional. If the ducts run between the rafters, it may make sense to insulate the underside of the roof and to reduce energy loss associated with duct leaks and duct conduction.

Cathedral ceilings require innovative approaches to insulation because they are within the living space. Rigid insulation, spray-foam, and blown-in products are available for ceiling applications and can be hidden behind finished ceiling materials.

Insulation Types

Insulation comes in different forms, at different prices, and requiring different skills or equipment. Each insulation type, however, is measured by its R-value, or resistance to heat flow. The higher the R-value, the more effective the insulation. R-value differs with material, thickness, and density.

Batts or rolls of mineral fibers are usually 3 1/2 to 6 inches thick and are a common do-it-yourself insulation material. The flexible rolls are available in widths that fit standard attic-joist spacings. On unfinished attic floors, the rolls are installed from perimeter to attic access at a depth reaching the tops of the joists. Add extra layers above the joist at right angles to the previous layer. The attic hatch also needs to be insulated. Walk only on joists or place walking boards across the joists to avoid compromising the insulation or putting a hole in the ceiling below.

Blown-in loose-fill insulation requires pumper equipment or the services of a professional installer. The loose fibers or fiber pellets are blown into attics, using air to get the correct density and thickness. Blown-in or loose-fill insulation is a good choice for odd-shaped spaces with multiple obstructions because it is loose, will fill spaces, and settles where there is an opening. Loose-fill insulation is popular in renovations because it can seek out available open space. In new construction, a vapor retarder may be needed on the living side to prevent moisture buildup in the insulation.

Foam insulation requires a professional applicator to spray the liquid polyurethane into place. While a bit messy, the foam forms a continuous insulating barrier, giving it the highest R-value compared to other insulation products. It’s good for around obstructions or irregularly shaped spaces, will never settle, and will fill all air leaks. Foam is a great choice for ceiling applications as it will adhere to the ceiling and allow for a finished ceiling product on top.

Rigid plastic insulation is made of pressed or extruded fiber or plastic materials formed into lightweight boards. The boards are flammable and require a thermal barrier. The boards may be faced with a reflective foil that reduces heat flow when installed next to an air space. This insulation is often used on unvented, low-slope roofs.

Radiant barriers are a different way to stop heat transfer. The barrier is a thin sheet of highly reflective material, such as aluminum, attached on one or both sides of kraft paper, cardboard, or another air-infiltration barrier. It can installed on top of existing attic insulation, attached to the bottom surfaces of attic trusses or rafters, or affixed directly to the underside of the roof deck. For new construction, it can be placed over the tops of the rafters before the roof deck is installed. While conventional insulation stops heat transfer through convection, a reflective radiant barrier reduces heat radiated across an air space next to the barrier. Radiant barriers are especially good for reducing heat intake and cool-air loss on sunny summer days.

Under-Roof and Attic Insulation - Insulation, HVAC

Insulation, Moisture, and Ventilation

Changing how attic or under-roof space is insulated will change the home’s relationship with heat, air, and moisture. With ductwork, it is important to correct any leaks that might cause heat and condensation buildup. Insulating the ducts is a good idea. Ventilating the insulated space is also critical. Work with an insulation specialist to choose the right insulation, installation, and ventilation for your space and your heating or cooling needs. Attic and under-roof spaces are ventilated differently in different climates. The key is to prevent moisture damage from venting in soffits or on the roof.

In all climates, airtight construction is important. Properly sealing the house may save more energy than adding insulation. Look for those forgotten areas, like knee walls and where the chimney runs through the attic. Loss of conditioned air also occurs through shrinkage and cracking of construction materials over time.

Properly install air and vapor retarders to make moisture less of a problem. Any venting must be part of a plan and cannot rely on random air leaks. Check soffit and attic vents to make sure they are clear of insulation, debris, or stored items.

Text by Maureen Blaney Flietner

2007 Renovate Your World


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